Bis(perfluoropinacolato)silane: A Neutral Silane Lewis Superacid Activates Si-F Bonds

Tetra-Amido Macrocyclic Ligand (TAML) at Silicon(IV): A Structurally Constrained, Water-Soluble Silicon Lewis Superacid

Tetracoordinate silicon species are typically tetrahedral, weak Lewis acids, and often sensitive to moisture. In this study, we present a tetra-amido macrocyclic ligand (TAML)-substituted Si(IV), isolated as its bis(pyridine) adduct. Due to structural constraint toward anti van't-Hof/Le Bel geometry, this compound exhibits Lewis superacidity and effectively catalyzes the hydroboration of pyridine. Kinetic and computational analyses of the catalytic cycle reveal that TAML-Si(IV) acts as a hydride transfer agent, and the hydrido silicate key intermediate is isolated. Notably, the Lewis acid is highly soluble (5 g/L) and long-term stable in water. Unlike previously described silicon-H2O adducts, the bound water becomes substantially acidified, reaching the Bro̷nsted superacidity range. A comparison of water affinity versus pKa lowering confirms our previous theory of the strength and the effect of Lewis acids. Overall, the compound's unlimited water compatibility and its mechanistically understood catalytic efficiency mark significant progress in applying structural constraint strategies for p-block element-based catalysis, while the acidification touches critical aspects of zeolite and silica surface chemistry.

Halogen, Chalcogen, Pnictogen, and Tetrel Bonding in Non-Covalent Organocatalysis: An Update

The use of noncovalent interactions based on electrophilic halogen, chalcogen, pnictogen, or tetrel centers in organocatalysis has gained noticeable attention. Herein, we provide an overview on the most important developments in the last years with a clear focus on experimental studies and on catalysts which act via such non-transient interactions.

A Sterically Accessible Monomeric Stibine Oxide Activates Organotetrel(IV) Halides, Including C-F and Si-F Bonds

Phosphine oxides and arsine oxides are common laboratory reagents with diverse applications that stem from the chemistry exhibited by these monomeric species. Stibine oxides are, in contrast, generally dimeric or oligomeric species because of the reactivity-quenching self-association of the highly polarized stiboryl (Sb=O/Sb+-O-) group. We recently isolated Dipp3SbO (Dipp = 2,6-diisopropylphenyl), the first example of a kinetically stabilized monomeric stibine oxide, which exists as a bench-stable solid and bears an unperturbed stiboryl group. Herein, we report the isolation of Mes3SbO (Mes = mesityl), in which the less bulky substituents maintain the monomeric nature of the compound but unlock access to a wider range of reactivity at the unperturbed stiboryl group relative to Dipp3SbO. Mes3SbO was found to be a potent Lewis base in the formation of adducts with the main-group Lewis acids PbMe3Cl and SnMe3Cl. The accessible Lewis acidity at the Sb atom results in a change in the reactivity with GeMe3Cl, SiMe3Cl, and CPh3Cl. With these species, Mes3SbO formally adds the E-Cl (E = Ge, Si, C) bond across the unsaturated stiboryl group to form a 5-coordinate stiborane. The biphilicity of Mes3SbO is sufficiently potent to activate even the C-F and Si-F bonds of C(p-MeOPh)3F and SiEt3F, respectively. These results mark a significant contribution to an increasingly rich literature on the reactivity of polar, unsaturated main-group motifs. Furthermore, these results highlight the utility of a kinetic stabilization approach to access unusual bonding motifs with unquenched reactivity that can be leveraged for small-molecule activation.

P(V)-bis(amidophenolate) ligand cooperation: stoichiometric CO-bond cleavage in aldehydes and ketones

The cooperation between a geometrically constrained, highly electrophilic phosphorus(V) center, and an electronically rich tetradentate bis(amidophenolate) ligand enables the cleavage of the CO bond from typical aldehydes and ketones delivering iminio phosphoramidate species. The amphiphilic nature of these products, which is demonstrated through their reaction with typical Lewis acids and bases, enables their use as a mild source of silylium cations from silanes, allowing the selective reductive coupling of aldehydes to ethers under catalytic conditions.

Syntheses and exploration of the catalytic activities of organotin(IV) compounds

Six organotin(IV) compounds (1-6) have been synthesized by reaction of the polydentate pro-ligands H3L and H2L, respectively, with the corresponding diorganotin chlorides. All of the compounds were characterized by FT-IR spectroscopy, 1H, 13C{1H}, and 119Sn (1H) NMR spectroscopy, HRMS spectrometry, and single-crystal X-ray diffraction. The solid-state structures show that all of the compounds are monomeric (except compound 3) and contain a penta-coordinated tin atom. Compound 3 is a dimer with two hexa-coordinated tin atoms. Compounds 1-3 contain a non-coordinated hydroxymethyl group. All of the compounds have been screened for their catalytic efficacy in the synthesis of 1,2 disubstituted benzimidazoles using o-phenylenediamine and aldehyde derivatives. It has been observed that both the Lewis acidic Sn(IV) centre and the hydroxymethyl group (hydrogen bond donor) catalyse the reactions with a product yield of up to 92%.

Predicting Lewis Acidity: Machine Learning the Fluoride Ion Affinity of p-Block-Atom-Based Molecules

"How strong is this Lewis acid?" is a question researchers often approach by calculating its fluoride ion affinity (FIA) with quantum chemistry. Here, we present FIA49k, an extensive FIA dataset with 48,986 data points calculated at the RI-DSD-BLYP-D3(BJ)/def2-QZVPP//PBEh-3c level of theory, including 13 different p-block atoms as the fluoride accepting site. The FIA49k dataset was used to train FIA-GNN, two message-passing graph neural networks, which predict gas and solution phase FIA values of molecules excluded from training with a mean absolute error of 14 kJ mol-1 (r2=0.93) from the SMILES string of the Lewis acid as the only input. The level of accuracy is notable, given the wide energetic range of 750 kJ mol-1 spanned by FIA49k. The model's value was demonstrated with four case studies, including predictions for molecules extracted from the Cambridge Structural Database and by reproducing results from catalysis research available in the literature. Weaknesses of the model are evaluated and interpreted chemically. FIA-GNN and the FIA49k dataset can be reached via a free web app (www.grebgroup.de/fia-gnn).

Mononuclear organogermanium(IV) catalysts for a [3 + 2] cycloaddition reaction

Three mononuclear Ge(IV) compounds, [(C6H5)2Ge(C13H8N2O4)] (1), [(C6H5)2Ge(C14H10N2O5)] (2), and [(C6H5)2Ge(C14H11NO3)] (3), have been synthesized by the reaction of pro-ligands H2L1 (C13H10N2O4), H2L2 (C14H12N2O5), and H2L3 (C14H13NO3) with (C6H5)2GeCl2 in the presence of triethylamine. All compounds were characterized by FT-IR spectroscopy and NMR spectroscopy. Single crystal X-ray diffraction analysis shows that the germanium(IV) atom exhibits a five-coordinated geometry in compounds 1 and 2. All compounds were screened as Lewis acid catalysts in the [3 + 2] cycloaddition reaction between sodium azide and various nitriles. The reactions resulted in the formation of 5-substituted 1H-tetrazoles with yields of up to 96%. Based on the experimental findings and DFT calculations, a plausible mechanism is proposed for the [3 + 2] cycloaddition reaction.

The Field of Main Group Lewis Acids and Lewis Superacids: Important Basics and Recent Developments

New developments in the field of Lewis acidity are highlighted, with the focus of novel Lewis acids and Lewis superacids of group 2, 13, 14, and 15 elements. Several important basics, illustrated by modern examples (classification of Donor-Acceptor (DA) complexes, amphoteric nature of any compound in terms of DA interactions, reorganization energies of main group Lewis acids and the role of the energies of frontier orbitals) are presented and discussed. It is emphasized that the Lewis acidity phenomena are general and play vital role in different areas of chemistry: from weak "atomophilic" interactions to the complexes of Lewis superacids.

A neutral germanium-centred hard and soft lewis superacid and its unique reactivity towards hydrosilanes

The germanium-centred Lewis superacid Ge(pinF)2 (1) was isolated as acetonitrile mono-adduct 1·MeCN and thoroughly characterized by NMR spectroscopy, X-ray crystallography and quantum chemical calculations. Ion abstraction and NMR experiments revealed the hard as well as soft Lewis superacidic nature of 1·MeCN. The title compound readily activates hydrosilanes such as Et3SiH, which is not feasible for its harder silicon homologue 2·MeCN, and even reacts with Et3SiF. The strongly coordinating acetonitrile could be abstracted by B(C6F5), giving the donor-free Ge(pinF)2 (1) and Si(pinF)2 (2) which are Lewis superacids. Unlike 1·MeCN, the donor-free 1 efficiently catalyses hydrosilylation of α-methylstyrene by Et3SiH. For this process, an inverse temperature dependence was observed, i.e. a complete conversion was achieved rapidly when the reaction was cooled to -35 °C, but the reaction stopped at elevated temperatures. Mechanistic investigations, including stoichiometric experiments and quantum chemical calculations, outlined the formation of germylene Ge(pinF) (3), which acts as the active catalyst. The germylene is formed by reductive elimination of the silylated pinacol from the hydrogermane intermediate, which is obtained by the initial reaction of 1 with Et3SiH. The inverse temperature dependence of the catalytic reaction could be explained by low entropy associated with the complexation of two cooperating germylenes and the substrates. With this example we introduce an in situ generated Lewis acidic germylene complex for catalytic hydrosilylation of olefins and again exemplify the great potential of main-group-element-based complexes in catalysis.

N-Coordinated tellurenium(II) and telluronium(IV) cations: synthesis, structure and hydrolysis

A set of N-coordinated tellurium(II) compounds containing either C,N-chelating ligands CNR (where CN = 2-(RNCH)C6H4, R = tBu or Dipp; Dipp = 2,6-iPr2C6H3) or N,C,N pincer ligands NCNR (where NCN = 2,6-(RNCH)2C6H4, R = tBu or Dipp) were synthesized. In the case of C,N-chelated compounds, the reaction of CNDippLi with Te(dtc)2 (where dtc = Et2NCS2) in a 1 : 1 molar ratio smoothly provided the carbamate CNDippTe(dtc) which upon treatment with 2 eq. of HCl provided the chloride CNDippTeCl. In contrast, the analogous conversion of NCNRLi with Te(dtc)2 surprisingly furnished ionic bromides [NCNRTe]Br as a result of the exchange of dtc by Br coming from nBuBr present in the reaction mixture. Furthermore, the reaction of CNDippTeCl or [NCNRTe]Br with silver salts AgX (X = OTf or SbF6) provided the expected tellurenium cations [CNDippTe]SbF6 and [NCNRTe]X. To further increase the Lewis acidity of the central atom, the oxidation of selected compounds with 1 eq. of SO2Cl2 was examined yielding stable compounds [CNtBuTeCl2]X and [NCNtBuTeCl2]X. The oxidation of the Dipp substituted compounds proved to be more challenging and an excess of SO2Cl2 was necessary to obtain the oxidized products [CNDippTeCl2]SbF6 and [NCNDippTeCl2]SbF6, which could solely be characterized in solution. Compounds [CNtBuTeCl2]OTf and [NCNtBuTeCl2]OTf were shown to undergo a controlled hydrolysis to the corresponding telluroxanes. All compounds were studied by multinuclear NMR spectroscopy in solution and for selected compounds solid state 125Te NMR spectroscopy and single-crystal X-ray diffraction analysis were performed. The Lewis acidity of the studied cations was examined by the Gutmann-Beckett method using Et3PO as the probing agent. The Te-N chalcogen bonding situation of selected compounds has also been examined computationally by a set of real-space bonding indicators.

Reversible C-H bond silylation with a neutral silicon Lewis acid

The silicon-carbon bond is a valuable linchpin for synthetic transformations. However, installing Si-C functionalities requires metalated C-nucleophiles, activated silicon reagents (silylium ions, silyl radicals, and silyl anions), or transition metal catalysis, and it occurs irreversibly. In contrast, spontaneous C-H silylations with neutral silanes leading to anionic silicates, and their reversible deconstruction, are elusive. Herein, the CH-bond silylation of heterocycles or a terminal alkyne is achieved by reaction with bis(perfluoro(N-phenyl-ortho-amidophenolato))silane and 1,2,2,6,6-pentamethylpiperidine. Computational and experimental insights reveal a frustrated Lewis pair (FLP) mechanism. Adding a silaphilic donor to the ammonium silicate products induces the reformation of the C-H bond, thus complementing previously known irreversible C-H bond silylation protocols. Interestingly, the FLP "activated" N-methylpyrrole exhibits "deactivated" features against electrophiles, while a catalytic functionalization is found to be effective only in the absence of a base.

Geometrically Constrained Organoboron Species as Lewis Superacids and Organic Superbases

This report unveils an advancement in the formation of a Lewis superacid (LSA) and an organic superbase by the geometrical deformation of an organoboron species towards a T-shaped geometry. The boron dication [2]2+ supported by an amido diphosphine pincer ligand features both a large fluoride ion affinity (FIA>SbF5 ) and hydride ion affinity (HIA>B(C6 F5 )3 ), which qualifies it as both a hard and soft LSA. The unusual Lewis acidic properties of [2]2+ are further showcased by its ability to abstract hydride and fluoride from Et3 SiH and AgSbF6 respectively, and effectively catalyze the hydrodefluorination, defluorination/arylation, as well as reduction of carbonyl compounds. One and two-electron reduction of [2]2+ affords stable boron radical cation [2]⋅+ and borylene 2, respectively. The former species has an extremely high spin density of 0.798e at the boron atom, whereas the latter compound has been demonstrated to be a strong organic base (calcd. pKBH + (MeCN)=47.4) by both theoretical and experimental assessment. Overall, these results demonstrate the strong ability of geometric constraining to empower the central boron atom.

Utilization of a Tris(carbene)borate Ligand for Umpolung Reactivity of a Nucleophilic Tin(II) Cation Salt

We show that a tris(carbene)borate (TCB) ligand, namely [PhB(^tBuIm)₃]^- ([PhB(^tBuIm)₃]^- = phenyltris(3-tert-butylimidazol-2-ylidene)borato), is capable of stabilizing an unprecedented nucleophilic Sn(II) cation salt. Unlike known Sn(II) cations, the strong electron-donating ability of [PhB(^tBuIm)₃]^- makes the cationic tin atom electron-rich, σ-donating yet slightly π-accepting, which allows for the ensuing facile oxidation with o-chloranil and S₈ as well as coordination with coinage metals. The former oxidations give the Sn(IV) cation salts, while the latter reactions produce the metal complexes. The electronic structures of these species are thoroughly probed by quantum chemical computations. These results uncover an added role for TCB ligands in isolating unprecedented p-block species.

Dihydrogen Activation with a Neutral, Intermolecular Silicon(IV)-Amine Frustrated Lewis Pair

The heterolytic cleavage of dihydrogen constitutes the hallmark reaction of frustrated Lewis pairs (FLP). While being well-established for planar Lewis acids, such as boranes or silylium ions, the observation of the primary H2 splitting products with non-planar Lewis acid FLPs remained elusive. In the present work, we report bis(perfluoro-N-phenyl-ortho-amidophenolato)silane and its application in dihydrogen activation to a fully characterized hydridosilicate. The strict design of the Lewis acid, the limited selection of the Lewis base, and the distinct reaction conditions emphasize the narrow tolerance to achieve this fascinating process with a tetrahedral Lewis acid.

Crystallographic evidence for a continuum and reversal of roles in primary-secondary interactions in antimony Lewis acids: applications in carbonyl activation

Primary and secondary interactions form the basis of substrate activation in Lewis-acid mediated catalysis, with most substrate activations occurring at the secondary binding site. We explore two series of antimony cations, [(NMe₂CH₂C₆H₄)(mesityl)Sb]⁺ (A) and [(NMe₂C₆H₄)(mesityl)Sb]⁺ (B), by coordinating ligands with varying nucleophilicity at the position trans to the N-donor. The decreased nucleophilicity of the incoming ligands leads to reversal from a primary bond to a secondary interaction in A, whereas a constrained N-coordination in B diminishes the border between primary and secondary bonding. Investigations on carbonyl olefin metathesis reactions and carbonyl reduction demonstrate increased reactivity of a Lewis acid when the substrate activation occurs at the primary binding site.

Bis(catecholato)germane: An Effective Catalyst for Friedel-Crafts Alkylation Reaction

Bis(catecholato)germane, [Ge(C6H4O2)2(H2O)2] (1) was synthesized by the reaction of catechol and germanium oxide in water according to a reported method. Complex 1 was characterized by FT-IR spectroscopy, NMR spectroscopy and...